HELP on New request is not allowed to start because it should come

I am getting the following error in my application:

New request is not allowed to start because it should come with valid
transaction descriptor.

This is happening under a very singular circustance:
I have just migrated to SQL server 2005 and ASP.NET 2.0.
It is not happening all the time. It happens occasionally in all my pages
that are working correctly, and once in a while, throw this kind of exceptio
n.
When it happens, it happens on the first Connection.Open of the page.
I am using Windows 2003 Enterprise.
Please, somebody help me. I trusted Microsoft, migrated all my systems and
now these errors are killing my application.
And the message is a kind of message that doesn`t give me any clue of what
is wrong.

Thanks a lot.